In general, chemical microreactors are provided with a microfluidic circuit, including microchannels. In the most advanced microfluidic devices the microchannels are buried in a substrate and/or in an epitaxial layer of a semiconductor chip. Substances to be processed, which are dispersed in a fluid medium, are supplied to one or more inlet reservoirs of the microfluidic circuit and are moved there through. Chemical reactions take place along the microfluidic circuit.
As is known, microfluidic devices may be exploited in a number of applications, and are particularly suited to be used as chemical microreactors. Thanks to the design flexibility allowed by semiconductor micromachining techniques, devices have been made that are capable of carrying out individual processing steps or even an entire chemical process.
For example, microfluidic devices are widely employed in biochemical processes, such as nucleic acid analysis. Such microreactors may also be called “Labs-On-Chip.” The discussion herein is simplified by focusing on nucleic acid analysis as an example of a biological molecule that can be analyzed using the various devices of the invention. However, the various devices may be used for any chemical or biological test, although typically molecule purification is substituted for amplification and detection methods vary according to the molecule being detected. For example, another common diagnostic involves the detection of a specific protein by binding to its antibody or by a specific enzymatic reaction. Lipids, carbohydrates, drugs and small molecules from biological fluids are processed in similar ways.
DNA amplification involves a series of enzyme-mediated reactions resulting in identical copies of the target nucleic acid. In particular, Polymerase Chain Reaction (PCR) is a cyclical process where the number of DNA molecules substantially doubles at every iteration, starting from a mixture comprising target DNA, enzymes (typically a DNA polymerase such as TAQ), primers, the four dNTPs, cofactor, and buffer.
During a cycle, double stranded DNA is first separated into single strands (denatured). Then the primers hybridize to their complementary sequences on either side of the target sequence. Finally, DNA polymerase extends each primer, by adding nucleotides that are complementary to the target strand. This doubles the DNA content and the cycle is repeated until sufficient DNA has been synthesized. RNA amplification is similar, but is typically preceded by copying the RNA into DNA.
Although PCR allows the production of millions of copies of the target sequence in few hours, in many cases its efficiency and speed might be improved by increasing the concentration of the reagents. Similarly, end-point detection of amplified DNA (amplicons) by hybridization is highly concentration dependant.
As already mentioned, in the most advanced microfluidic devices the channels are “buried” in a substrate and/or in an epitaxial layer of a semiconductor chip. However, processes for manufacturing microfluidic devices with buried channels are quite complicated. In particular, several steps are required once the buried channels have been completed and alignment of subsequent masks is often critical. Usually, additional steps are required to reveal the alignment signs of the wafer being processed, which would otherwise be hidden.
A known technique is described in “PROCEEDINGS OF THE IEEE,” Vol. 86, No. 8, August 1998, page 1632, and essentially envisages the creation of a cavity or air gap by means of anisotropic chemical etches made using potassium hydroxide (KOH), tetramethyl ammonium hydroxide (TMAH), etc., and employing a sacrificial polycrystalline-silicon layer.
This technique is schematically illustrated in FIGS. 1a-1c, and essentially involves the deposition and definition, using a special mask, of a sacrificial polycrystalline-silicon layer 5 on the top surface of the substrate 1, deposition of a silicon-nitride (Si3N4) layer 6 above the sacrificial polycrystalline-silicon layer 5 (FIG. 1a), and then the carrying-out of an anisotropic etch of the substrate 1 through an opening 7 made in the silicon-nitride layer 6 (FIG. 1b). By means of the anisotropic etch, the sacrificial polycrystalline-silicon layer 5 and part of the substrate 1 are thus removed, and a cavity or air-gap 8 is obtained having a roughly triangular cross section, which is separated from the outside environment by a diaphragm 9 consisting of the portion of the silicon-nitride layer 6 overlying the cavity 8 (FIG. 1c), and on which the inductor can subsequently be made.